Compact hyperbolic Coxeter n-polytopes with n + 3 facets

Compact hyperbolic Coxeter n-polytopes with n + 3 facets Pavel Tumarkin Independent University of Moscow B. Vlassievskii 11, 11900 Moscow, Russia pasha@mccme.ru Submitted: Apr 3, 007; Accepted: Sep 30, 007; Published: Oct 5, 007 Mathematics Subject Classifications: 51M0, 51F15, 0F55 Abstract We use methods of combinatorics of polytopes together with geometrical and computational ones to obtain the complete list of compact hyperbolic Coxeter n- polytopes with n + 3 facets, 4 n 7. Combined with results of Esselmann this gives the classification of all compact hyperbolic Coxeter n-polytopes with n + 3 facets, n 4. Polytopes in dimensions and 3 were classified by Poincaré and Andreev. 1 Introduction A polytope in the hyperbolic space H n is called a Coxeter polytope if its dihedral angles are all integer submultiples of π. Any Coxeter polytope P is a fundamental domain of the discrete group generated by reflections in the facets of P. There is no complete classification of compact hyperbolic Coxeter polytopes. Vinberg [V1] proved there are no such polytopes in H n, n 30. Examples are known only for n 8 (see [B1], [B]). In dimensions and 3 compact Coxeter polytopes were completely classified by Poincaré [P] and Andreev [A]. Compact polytopes of the simplest combinatorial type, the simplices, were classified by Lannér [L]. Kaplinskaja [K] (see also [V]) listed simplicial prisms, Esselmann [E] classified the remaining compact n-polytopes with n + facets. In the paper [ImH] Im Hof classified polytopes that can be described by Napier cycles. These polytopes have at most n + 3 facets. Concerning polytopes with n + 3 facets, Esselmann proved the following theorem ([E1, Th. 5.1]): Partially supported by grants MK-690.006.1, NSh-5666.006.1, INTAS grant YSF-06-000014- 5916, and RFBR grant 07-01-00390-a the electronic journal of combinatorics 14 (007), #R69 1

Let P be a compact hyperbolic Coxeter n-polytope bounded by n+3 facets. Then n 8; if n = 8, then P is the polytope found by Bugaenko in [B]. This polytope has the following Coxeter diagram: In this paper, we expand the technique derived by Esselmann in [E1] and [E] to complete the classification of compact hyperbolic Coxeter n-polytopes with n + 3 facets. The aim is to prove the following theorem: Main Theorem. Tables 4.8 4.11 contain all Coxeter diagrams of compact hyperbolic Coxeter n-polytopes with n + 3 facets for n 4. The paper is organized as follows. In Section we recall basic definitions and list some well-known properties of hyperbolic Coxeter polytopes. We also emphasize the connection between combinatorics (Gale diagram) and metric properties (Coxeter diagram) of hyperbolic Coxeter polytope. In Section 3 we recall some technical tools from [V1] and [E1] concerning Coxeter diagrams and Gale diagrams, and introduce notation suitable for investigating of large number of diagrams. Section 4 is devoted to the proof of the main theorem. The most part of the proof is computational: we restrict the number of Coxeter diagrams in consideration, and use a computer check after that. The bulk is to find an upper bound for the number of diagrams, and then to reduce the number to make the computation short enough. This paper is a completely rewritten part of my Ph.D. thesis (004) with several errors corrected. I am grateful to my advisor Prof. E. B. Vinberg for his help. I am also grateful to Prof. R. Kellerhals who brought the papers of F. Esselmann and L. Schlettwein to my attention, and to the referee for useful suggestions. Hyperbolic Coxeter polytopes and Gale diagrams In this section we list essential facts concerning hyperbolic Coxeter polytopes, Gale diagrams of simple polytopes, and Coxeter diagrams we use in this paper. Proofs, details and definitions in general case may be found in [G] and [V]. In the last part of this section we present the main tools used for the proof of the main theorem. We write n-polytope instead of n-dimensional polytope for short. By facet we mean a face of codimension one..1 Gale diagrams An n-polytope is called simple if any its k-face belongs to exactly n k facets. Proposition. implies that any compact hyperbolic Coxeter polytope is simple. From now on we consider simple polytopes only. the electronic journal of combinatorics 14 (007), #R69

Every combinatorial type of simple n-polytope with d facets can be represented by its Gale diagram G. This consists of d points a 1,..., a d on the (d n )-dimensional unit sphere in R d n 1 centered at the origin. The combinatorial type of a simple convex polytope can be read off from the Gale diagram in the following way. Each point a i corresponds to the facet f i of P. For any subset J of the set of facets of P the intersection of facets {f j j J} is a face of P if and only if the origin is contained in the interior of conv{a j j / J}. The points a 1,..., a d S d n compose a Gale diagram of some n-dimensional polytope P with d facets if and only if every open half-space H + in R d n 1 bounded by a hyperplane H through the origin contains at least two of the points a 1,..., a d. We should notice that the definition of Gale diagram introduced above is dual to the standard one (see, for example, [G]): usually Gale diagram is defined in terms of vertices of polytope instead of facets. Notice also that the definition above concerns simple polytopes only, and it takes simplices out of consideration: usually one means the origin of R 1 with multiplicity n + 1 by the Gale diagram of an n-simplex, however we exclude the origin since we consider simple polytopes only, and the origin is not contained in G for any simple polytope except simplex. We say that two Gale diagrams G and G are isomorphic if the corresponding polytopes are combinatorially equivalent. If d = n + 3 then the Gale diagram of P is two-dimensional, i.e. nodes a i of the diagram lie on the unit circle. A standard Gale diagram of simple n-polytope with n + 3 facets consists of vertices v 1,..., v k of regular k-gon (k is odd) in R centered at the origin which are labeled according to the following rules: 1) Each label is a positive integer, the sum of labels equals n + 3. ) The vertices that lie in any open half-space bounded by a line through the origin have labels whose sum is at least two. Each point v i with label µ i corresponds to µ i facets f i,1,..., f i,µi of P. For any subset J of the set of facets of P the intersection of facets {f j,γ (j, γ) J} is a face of P if and only if the origin is contained in the interior of conv{v j (j, γ) / J}. It is easy to check (see, for example, [G, Sec. 6.3]) that any two-dimensional Gale diagram is isomorphic to some standard diagram. Two simple n-polytopes with n + 3 facets are combinatorially equivalent if and only if their standard Gale diagrams are congruent.. Coxeter diagrams Any Coxeter polytope P can be represented by its Coxeter diagram. An abstract Coxeter diagram is a one-dimensional simplicial complex with weighted edges, where weights are either of the type cos π for some integer m 3 or positive real m numbers no less than one. We can suppress the weights but indicate the same information by labeling the edges of a Coxeter diagram in the following way: the electronic journal of combinatorics 14 (007), #R69 3

if the weight equals cos π then the nodes are joined by either an (m )-fold edge or a m simple edge labeled by m; if the weight equals one then the nodes are joined by a bold edge; if the weight is greater than one then the nodes are joined by a dotted edge labeled by its weight. A subdiagram of Coxeter diagram is a subcomplex with the same as in Σ. The order Σ is the number of vertices of the diagram Σ. If Σ 1 and Σ are subdiagrams of a Coxeter diagram Σ, we denote by Σ 1, Σ a subdiagram of Σ spanned by all nodes of Σ 1 and Σ. We say that a node of Σ attaches to a subdiagram Σ 1 Σ if it is joined with some nodes of Σ 1 by edges of any type. Let Σ be a diagram with d nodes,...,u d. Define a symmetric d d matrix Gr(Σ) in the following way: g ii = 1; if two nodes u i and u j are adjacent then g ij equals negative weight of the edge u i u j ; if two nodes u i and u j are not adjacent then g ij equals zero. By signature and determinant of diagram Σ we mean the signature and the determinant of the matrix Gr(Σ). An abstract Coxeter diagram Σ is called elliptic if the matrix Gr(Σ) is positive definite. A Coxeter diagram Σ is called parabolic if the matrix Gr(Σ) is degenerate, and any subdiagram of Σ is elliptic. Connected elliptic and parabolic diagrams were classified by Coxeter [C]. We represent the list in Table.1. A Coxeter diagram Σ is called a Lannér diagram if any subdiagram of Σ is elliptic, and the diagram Σ is neither elliptic nor parabolic. Lannér diagrams were classified by Lannér [L]. We represent the list in Table.. A diagram Σ is superhyperbolic if its negative inertia index is greater than 1. By a simple (resp., multiple) edge of Coxeter diagram we mean an (m )-fold edge where m is equal to (resp., greater than) 3. The number m is called the multiplicity of a multiple edge. Edges of multiplicity greater than 3 we call multi-multiple edges. If an edge u i u j has multiplicity m (i.e. the corresponding facets form an angle π ), we m write [u i, u j ] = m. A Coxeter diagram Σ(P ) of Coxeter polytope P is a Coxeter diagram whose matrix Gr(Σ) coincides with Gram matrix of outer unit normals to the facets of P (referring to the standard model of hyperbolic n-space in R n,1 ). In other words, nodes of Coxeter diagram correspond to facets of P. Two nodes are joined by either an (m )-fold edge or an m-labeled edge if the corresponding dihedral angle equals π. If the corresponding facets m are parallel the nodes are joined by a bold edge, and if they diverge then the nodes are joined by a dotted edge (which may be labeled by hyperbolic cosine of distance between the hyperplanes containing these facets). If Σ(P ) is the Coxeter diagram of P then nodes of Σ(P ) are in one-to-one correspondence with elements of the set I = {1,..., d}. For any subset J I denote by Σ(P ) J the subdiagram of Σ(P ) that consists of nodes corresponding to elements of J. the electronic journal of combinatorics 14 (007), #R69 4

Table.1: Connected elliptic and parabolic Coxeter diagrams are listed in left and right columns respectively. A n (n 1) Ã 1 Ã n (n ) B n = C n Bn (n 3) (n ) Cn (n ) D n (n 4) Dn (n 4) PSfrag G (m) replacements m G F 4 F4 E 6 Ẽ 6 E 7 Ẽ 7 E 8 Ẽ 8 H 3 H 4.3 Hyperbolic Coxeter polytopes In this section by polytope we mean a (probably non-compact) intersection of closed half-spaces. Proposition.1 ([V], Th..1). Let Gr = (g ij ) be indecomposable symmetric matrix of signature (n, 1), where g ii = 1 and g ij 0 if i j. Then there exists a unique (up to isometry of H n ) convex polytope P H n whose Gram matrix coincides with Gr. Let Gr be the Gram matrix of the polytope P, and let J I be a subset of the set of facets of P. Denote by Gr J the Gram matrix of vectors {e i i J}, where e i is outward unit normal to the facet f i of P (i.e. Gr J = Gr(Σ(P ) J )). Denote by J the number of elements of J. the electronic journal of combinatorics 14 (007), #R69 5

Table.: Lannér diagrams. order PSfrag replacements k 3 m l diagrams ( k, l, m <, 1 k + 1 l + 1 m < 1) 4 5 Proposition. ([V], Th. 3.1). Let P H n be an acute-angled polytope with Gram matrix Gr, and let J be a subset of the set of facets of P. The set q = P i J f i is a face of P if and only if the matrix Gr J is positive definite. Dimension of q is equal to n J. Notice that Prop.. implies that the combinatorics of P is completely determined by the Coxeter diagram Σ(P ). Let A be a symmetric matrix whose non-diagonal elements are non-positive. A is called indecomposable if it cannot be transformed to a block-diagonal matrix via simultaneous permutations of columns and rows. We say A to be parabolic if any indecomposable component of A is positive semidefinite and degenerate. For example, a matrix Gr(Σ) for any parabolic diagram Σ is parabolic. Proposition.3 ([V], cor. of Th. 4.1, Prop. 3. and Th. 3.). Let P H n be a compact Coxeter polytope, and let Gr be its Gram matrix. Then for any J I the matrix Gr J is not parabolic. the electronic journal of combinatorics 14 (007), #R69 6

Corollary.1 reformulates Prop..3 in terms of Coxeter diagrams. Corollary.1. Let P H n be a compact Coxeter polytope, and let Σ be its Coxeter matrix. Then any non-elliptic subdiagram of Σ contains a Lannér subdiagram. Proposition.4 ([V], Prop. 4.). A polytope P in H n is compact if and only if it is combinatorially equivalent to some compact convex n-polytope. The main result of paper [FT] claims that if P is a compact hyperbolic Coxeter n- polytope having no pair of disjoint facets, then P is either a simplex or one of the seven polytopes with n + facets described in [E1]. As a corollary, we obtain the following proposition. Proposition.5. Let P H n be a compact Coxeter polytope with at least n + 3 facets. Then P has a pair of disjoint facets..4 Coxeter diagrams, Gale diagrams, and missing faces Now, for any compact hyperbolic Coxeter polytope we have two diagrams which carry the complete information about its combinatorics, namely Gale diagram and Coxeter diagram. The interplay between them is described by the following lemma, which is a reformulation of results listed in Section.3 in terms of Coxeter diagrams and Gale diagrams. Lemma.1. A Coxeter diagram Σ with nodes {u i i = 1,..., d} is a Coxeter diagram of some compact hyperbolic Coxeter n-polytope with d facets if and only if the following two conditions hold: 1) Σ is of signature (n, 1, d n 1); ) there exists a (d n 1)-dimensional Gale diagram with nodes {v i i = 1,..., d} and one-to-one map ψ : {u i i = 1,..., d} {v i i = 1,..., d} such that for any J {1,..., d} the subdiagram Σ J of Σ is elliptic if and only if the origin is contained in the interior of conv{ψ(v i ) i / J}. Let P be a simple polytope. The facets f 1,..., f m of P compose a missing face of P if m f i = but any proper subset of {f 1,..., f m } has a non-empty intersection. i=1 Proposition.6 ([FT], Lemma ). Let P be a simple d-polytope with d+k facets {f i }, let G = {a i } S k be a Gale diagram of P, and let I {1,..., d + k}. Then the set M I = {f i i I} is a missing face of P if and only if the following two conditions hold: (1) there exists a hyperplane H through the origin separating the set M I = {a i i I} from the remaining points of G; () for any proper subset J I no hyperplane through the origin separates the set M J = {a i i J} from the remaining points of G. the electronic journal of combinatorics 14 (007), #R69 7

Remark. Suppose that P is a compact hyperbolic Coxeter polytope. The definition of missing face (together with Cor..1) implies that for any Lannér subdiagram L Σ(P ) the facets corresponding to L compose a missing face of P, and any missing face of P corresponds to some Lannér diagram in Σ(P ). Now consider a compact hyperbolic Coxeter n-polytope P with n + 3 facets with standard Gale diagram G (which is a k-gon, k is odd) and Coxeter diagram Σ. Denote by Σ i,j a subdiagram of Σ corresponding to j i + 1 (mod k) consecutive nodes a i,..., a j of G (in the sense of Lemma.1). If i = j, denote Σ i,i by Σ i. The following lemma is an immediate corollary of Prop..6. Lemma.. For any i {0,..., k 1} a diagram Σ i+1,i+ k 1 Lannér diagrams contained in Σ are of this type. is a Lannér diagram. All It is easy to see that the collection of missing faces completely determines the combinatorics of P. In view of Lemma. and the remark above, this means that in Lemma.1 PSfrag for replacements given Coxeter diagram we need to check the signature and correspondence of Lannér diagrams to missing faces of some Gale diagram. Example. Suppose that there exists a compact hyperbolic Coxeter polytope P with standard Gale diagram G shown in Fig..1(a). What can we say about Coxeter diagram Σ = Σ(P )? 1 u 5 8 8 u 3 u 6 u 7 (a) 1 1 (b) u Figure.1: (a) A standard Gale diagram G and (b) a Coxeter diagram of one of polytopes with Gale diagram G The sum of labels of nodes of Gale diagram G is equal to 7, so P is a 4-polytope with 7 facets. Thus, Σ is spanned by nodes,..., u 7, and its signature equals (4, 1, ). Further, G is a pentagon. By Lemma., Σ contains exactly 5 Lannér diagrams, namely, u, u, u 3,, u 3,, u 5, u 5, u 6, u 7, and u 6, u 7,. Now consider the Coxeter diagram Σ shown in Fig..1(b). Assigning label 1 + to the dotted edge of Σ, we obtain a diagram of signature (4, 1, ) (this may be shown by direct calculation). Therefore, there exist 7 vectors in H 4 with Gram matrix Gr(Σ). It is easy to see that Σ contains exactly 5 Lannér diagrams described above. Thus, Σ is a Coxeter diagram of some compact 4-polytope with Gale diagram G. Of course, Σ is just an example of a Coxeter diagram satisfying both conditions of Lemma.1 with respect to given Gale diagram G. In the next two sections we will show how to list all compact hyperbolic Coxeter polytopes of given combinatorial type. the electronic journal of combinatorics 14 (007), #R69 8

3 Technical tools From now on by polytope we mean a compact hyperbolic Coxeter n-polytope with n + 3 facets, and we deal with standard Gale diagrams only. 3.1 Admissible Gale diagrams Suppose that there exists a compact hyperbolic Coxeter polytope P with k-angled Gale diagram G. Since the maximal order of Lannér diagram equals five, Lemma. implies that the sum of labels of k 1 consecutive nodes of Gale diagram does not exceed five. On the other hand, by Lemma.5, P has a missing face of order two. This is possible in two cases only: either G is a pentagon with two neighboring vertices labeled by 1, or G is a triangle one of whose vertices is labeled by (see Prop..6). Table 3.1 contains all Gale diagrams satisfying one of two conditions above with at least 7 and at most vertices, i.e. Gale diagrams that may correspond to compact hyperbolic Coxeter n-polytopes with n + 3 facets for 4 n 7. 3. Admissible arcs Let P be an n-polytope with n + 3 facets and let G be its k-angled Gale diagram. By Lemma., for any i {0,..., k 1} the diagram Σ i+1,i+ k 1 is a Lannér diagram. Denote by x 1,..., x l k 1, l k an arc of length l of G that consists of l consecutive nodes with labels x 1,..., x l. By writing J = x 1,..., x l k 1 we mean that J is the set of facets of P corresponding to these nodes of G. The index k 1 means that for any k 1 consecutive nodes of the arc (i.e. for any arc I = x i+1,..., x i+ k 1 ) the subdiagram Σ k 1 I of Σ(P ) corresponding to these nodes is a Lannér diagram (i.e. I is a missing face of P ). By Cor..1, any diagram Σ J Σ(P ) corresponding to an arc J = x 1,..., x l k 1 satisfies the following property: any subdiagram of Σ J containing no Lannér diagram is elliptic. Clearly, any subdiagram of Σ(P ) containing at least one Lannér diagram is of signature (k, 1) for some k n. As it is shown in [E1], for some arcs J there exist a few corresponding diagrams Σ J only. In the following lemma, we recall some results of Esselmann [E1] and prove similar facts concerning some arcs of Gale diagrams listed in Table 3.1. This will help us to restrict the number of Coxeter diagrams that may correspond to some of Gale diagrams listed in Table 3.1. Lemma 3.1. The diagrams presented in the middle column of Table 3. are the only diagrams that may correspond to arcs listed in the left column. Proof. At first, notice that for any J as above (i.e. J consists of several consecutive nodes of Gale diagram) the diagram Σ J must be connected. This follows from the fact that any Lannér diagram is connected, and that Σ J is not superhyperbolic. the electronic journal of combinatorics 14 (007), #R69 9

replacements Table 3.1: Gale diagrams that may correspond to compact Coxeter polytopes (see Sec- 3.1) 1tion replacements 3n = 4 4 3 3 1 1 G 3 G 11311 4 1 1 1 1 replacements G 111 G 111 1 1 1 1 1 3 G 3 G 11311 G 111 G 111 4 G 3 n = 5 G 11311 4 3 3 4 G 111 G 1 1 1 1 1 111 replacements 1 G 3 G 11311 G 111 G 111 3 n = 6 4 5 n = 7 3 3 1 1 1 1 1 1 1 G 4 G 33 G 1311 G 1311 G 11411 G 11 4 4 3 3 1 1 1 3 3 1 1 1 1 1 1 G 5 G 34 G 1411 G 131 G 311 G 13131 1 3 5 4 1 1 3 3 3 4 4 3 1 1 1 G 35 G 44 G 31411 G 1331 Now we restrict our considerations to items 8 11 only. For none of these J the diagram Σ J contains a Lannér diagram of order or 3. Since Σ J is connected and does not contain parabolic subdiagrams, this implies that Σ J does not contain neither dotted nor multimultiple edges. Thus, we are left with finitely many possibilities only, that allows us to use a computer check: there are several (from 5 to 7) nodes, some of them joined by edges of multiplicity at most 3. We only need to check all possible diagrams for the number of the electronic journal of combinatorics 14 (007), #R69

Table 3.: Possible diagrams Σ J for some arcs J. White nodes correspond to endpoints of arcs having multiplicity one 1 J all possibilities for Σ J reference (if any) x, y 1, x 4, y 3 [E1], Lemma 4.7 1, 4, 1 [E1], Lemma 5.3 3 3,, [E1], Lemma 5.7 4 4, 1, 3 [E1], Lemma 5.9 5 3, 1, 4, 1 [E1], Folgerung 5. 6, 3, [E1], Lemma 5.1 7 3, PSfrag, 3 replacements [E1], Lemma 5.1 3, 4 4,5 3, 4, 5 8 PSfrag 1, 3, replacements 1 PSfrag replacements 4,5 9 1, 3, 3, 4 3, 4, 5 PSfrag replacements 3,4 PSfrag replacements PSfrag replacements 3,4 4, 5 3, 4, 5 4,5 3, 44, 5 4, 5 3,4,5 3, 4 4,5 3, 4, 5,, 11 3, 1, 3 Lannér diagrams of all orders and for parabolic subdiagrams. Namely, in items 8, and 11 we look for diagrams of order 5, 6 and 7 containing exactly Lannér subdiagrams of order 4 (and containing neither other Lannér diagrams nor parabolic subdiagrams), and in the electronic journal of combinatorics 14 (007), #R69 11

item 9 we look for diagrams of order 6 containing exactly one Lannér subdiagram of order 4 and exactly one Lannér diagram of order 5. Notice also that we do not need to check the signature of obtained diagrams: all them are certainly non-elliptic, and since any of them contains exactly two Lannér diagrams which have at least one node in common, by excluding this node we obtain an elliptic diagram. However, the computation described above is really huge. In what follows we describe case-by-case how to reduce these computations to a few minutes of hand-calculations. Item 8 (J = 1, 3, 1 ). We may consider Σ J as a Lannér diagram L of order 4 together with one vertex attached to L to compose a unique additional Lannér diagram which should be of order 4, too. There are 9 possibilities for L only (Table.). Item 9 (J = 1, 3, ). The considerations follow the preceding ones, but we take as L a Lannér diagram of order 5. Again, there are few possibilities for L only (namely five: see Table.). Item (J =,, ). Again, Σ J contains a Lannér diagram L of order 4. One of the two remaining nodes of Σ J must be attached to L. Denote this node by v. The diagram L, v Σ J consists of five nodes and contains a unique Lannér diagram which is of order 4. All such diagrams are listed in [E1, Lemma 3.8] (see the first two rows of Tabelle 3, the case N F = 1, L F = 4). We reproduce this list in Table 3.3. Table 3.3: One of these diagrams should be contained in Σ J for J =,, PSfrag replacements u 7 One can see that there are six possibilities only. Now to each of them we attach the remaining node to compose a unique new Lannér diagram which should be of order 4. Item 11 (J = 3, 1, 3 ). The considerations are very similar to the preceding case. Σ J contains a Lannér diagram L of order 4. One of the three remaining nodes of Σ J must be attached to L. Denote this node by v. Now, one of the two remaining nodes attaches to L, v Σ J. Denote it by u. The diagram L, v, u Σ J consists of six nodes and contains a unique Lannér diagram which is of order 4. All such diagrams are listed in [E1, Lemma 3.8] (see Tabelle 3, the first two rows of page 7, the case N F =, L F = 4). We reproduce this list in Table 3.4. There are five possibilities only. As above, we attach to each of them the remaining node to compose a unique new Lannér diagram which should be of order 4. the electronic journal of combinatorics 14 (007), #R69 1

Table 3.4: One of these diagrams should be contained in Σ J for J = 3, 1, 3 PSfrag replacements u 7 3.3 Local determinants In this section we list some tools derived in [V1] to compute determinants of Coxeter diagrams. We will use them to show that some (infinite) series of Coxeter diagrams are superhyperbolic. Let Σ be a Coxeter diagram, and let T be a subdiagram of Σ such that det(σ\t ) 0. A local determinant of Σ on a subdiagram T is det(σ, T ) = det Σ det(σ\t ). Proposition 3.1 ([V1], Prop. 1). If a Coxeter diagram Σ consists of two subdiagrams Σ 1 and Σ having a unique vertex v in common, and no vertex of Σ 1 \v attaches to Σ \v, then det(σ, v) = det(σ 1, v) + det(σ, v) 1. Proposition 3. ([V1], Prop. 13). If a Coxeter diagram Σ is spanned by two disjoint subdiagrams Σ 1 and Σ joined by a unique edge v 1 v of weight a, then det(σ, v 1, v ) = det(σ 1, v 1 ) det(σ, v ) a. Denote by L p,q,r a Lannér diagram of order 3 containing subdiagrams of the dihedral groups G (p), G (q) and G (r). Let v be the vertex of L p,q,r that does not belong to G (r), see Fig. 3.1. Denote by D (p, q, r) the local determinant det(l p,q,r, v). It is easy to check (see e.g. [V1]) that D (p, q, r) = 1 cos (π/p) + cos (π/q) + cos(π/p) cos(π/q) cos(π/r) sin. (π/r) Notice that D (p, q, r) is an increasing function on each of p, q, r tending to infinity while r tends to infinity. 4 Proof of the Main Theorem The plan of the proof is the following. First, we show that there is only a finite number of combinatorial types (or Gale diagrams) of polytopes we are interested in, and we list the electronic journal of combinatorics 14 (007), #R69 13

PSfrag replacements v p r q Figure 3.1: Diagram L p,q,r these Gale diagrams. This was done in Table 3.1. For any Gale diagram from the list we should find all Coxeter polytopes of given combinatorial type. For that, we try to find all Coxeter diagrams with the same structure of Lannér diagrams as the structure of missing faces of the Gale diagram is, and then check the signature. Our task is to be left with finite number of possibilities for each of Gale diagrams, and use a computer after that. Some computations involve a large number of cases, but usually it takes a few minutes of computer s thought. In cases when it is possible to hugely reduce the computations by better estimates we do that, but we follow that by long computations to avoid mistakes. Lemma 4.1. The following Gale diagrams do not correspond to any hyperbolic Coxeter polytope: G 34, G 311, G 13131, G 35, G 44, G 31411. Proof. The statement follows from Lemma 3.1. Indeed, the diagram G 34 contains an arc J = 3, 4 1. The corresponding Coxeter diagram Σ J should be of order 7, should contain exactly two Lannér diagrams of order 3 and 4 which do not intersect, and should have negative inertia index at most one. Item 1 of Table 3. implies that there is no such Coxeter diagram Σ J. Thus, G 34 is not a Gale diagram of any hyperbolic Coxeter polytope. Similarly, Item 1 of Table 3. also implies the statement of the lemma for diagrams G 35 and G 44. Item 3 implies the statement for G 311, Item 11 implies the statement for G 13131, and Item 5 implies the statement for the diagram G 31411. In what follows we check the 14 remaining Gale diagrams case-by-case. We start from larger dimensions. 4.1 Dimension 7 In dimension 7 we have only one diagram to consider, namely G 1331. Lemma 4.. There are no compact hyperbolic Coxeter 7-polytopes with facets. Proof. Suppose that there exists a compact hyperbolic Coxeter polytope P with Gale diagram G 1331. This Gale diagram contains an arc J = 3,, 3. According to Lemma 3.1 (Item 7 of Table 3.) and Lemma., the Coxeter diagram Σ of P consists of a subdiagram Σ J shown in Fig. 4.1, and two nodes u 9, u joined by a dotted edge. By Lemma.1, the subdiagrams u,, u, u 3 and u 6, u 7, u 8, u 9 are Lannér diagrams, and no other the electronic journal of combinatorics 14 (007), #R69 14

u u 3 u 5 u 6 u 7 u 8 Figure 4.1: A unique diagram Σ J for J = 3,, 3 Lannér subdiagram of Σ contains u 9 or u. In particular, Σ does not contain Lannér subdiagrams of order 3. Consider the diagram Σ = Σ J, u 9. It is connected and contains neither Lannér diagrams of order or 3, nor parabolic diagrams. Therefore, Σ does not contain neither Sfrag replacements PSfrag replacements dotted nor multi-multiple edges. Moreover, by the same reason the node u 9 may attach to nodes, u, u 7 and u 8 by simple edges only. It follows that there are finitely many possibilities for the diagram Σ. Further, since the diagram Σ defines a collection of 9 vectors in 8-dimensional space R 7,1, the determinant of Σ is equal to zero. A few seconds computer check shows that the only diagrams satisfying conditions listed in this paragraph are the following ones: u u 3 u 5 u 6 u 7 u 8 u u 3 u 5 u 6 u 7 u 8 u 9 However, the left one contains a Lannér diagram u,, u 9,, u 5, and the right one contains a Lannér diagram u 7, u 8, u 9, u 5,, which is impossible since u 9 does not belong to any Lannér diagram of order 5. u 9 4. Dimension 6 In dimension 6 we are left with three diagrams, namely G 5, G 1411, and G 131. Lemma 4.3. There is only one compact hyperbolic Coxeter polytope with Gale diagram G 131. PSfrag Its Coxeter replacements diagram is the lowest one shown in Table 4.9. Proof. Let P be a compact hyperbolic Coxeter polytope with Gale diagram G 131. This Gale diagram contains an arc J =, 3,. According to Lemma 3.1 (Item 6 of Table 3.) and Lemma., the Coxeter diagram Σ of P consists of a subdiagram Σ J shown in Fig. 4., and two nodes u 8, u 9 joined by a dotted edge. By Lemma.1, the subdiagrams u 8,, u u u 3 u 5 u 6 u 7 Figure 4.: A unique diagram Σ J for J =, 3, and u 6, u 7, u 9 are Lannér diagrams, and no other Lannér subdiagram of Σ contains u 8 or u 9. So, we need to check possible multiplicities of edges incident to u 8 and u 9. the electronic journal of combinatorics 14 (007), #R69 15

Consider the diagram Σ = Σ J, u 8. It is connected, contains neither Lannér diagrams of order nor parabolic diagrams, and contains a unique Lannér diagram of order 3, namely u 8,, u. Therefore, Σ does not contain dotted edges, and the only multimultiple edge that may appear should join u 8 and. On the other hand, the signature of Σ J is (6, 1). This implies that the corresponding vectors in R 6,1 form a basis, so the multiplicity of the edge u 8 is completely determined by multiplicities of edges joining u 8 with the remaining nodes of Σ J. Since these edges are neither dotted nor multi-multiple, we are left with a finite number of possibilities only. We may reduce further computations observing that u 8 does not attach to, u 5, u 6, u 7 (since the diagram u 8,, u 5, u 6, u 7 should be elliptic), and that multiplicities of edges Sfrag replacements u 8 u and u 8 u 3 are at mostpsfrag two andreplacements one respectively. Therefore, we have the following possibilities: [u 8, u ] =, 3, 4, and, independently, [u 8, u 3 ] =, 3. For each of these six cases we should attach the node u 8 to satisfying the condition det Σ = 0. An explicit calculation shows that there are two diagrams listed below. u u 3 u 5 u 6 u 7 u u 3 u 5 u 6 u 7 u 8 The left one contains a Lannér diagram, u 8, u 3,, u 5, which is impossible. At the same time, the right one contains exactly Lannér diagrams prescribed by Gale diagram. Similarly, the node u 9 may be attached to Σ J in a unique way, i.e. by a unique edge u 9 u 6 of multiplicity two. Thus, Σ must look like the diagram shown in Fig. 4.3. Now we write down the determinant of Σ as a quadratic polynomial of the weight d of the dotted edge. An easy computation shows that det Σ = 5 3 u 8 ( d ( 5 + )). The signature of Σ for d = 5 + is equal to (6, 1, ), so we obtain that this diagram corresponds to a Coxeter polytope. PSfrag replacements Figure 4.3: Coxeter diagram of a unique Coxeter polytope with Gale diagram G 131 Lemma 4.4. There are two compact hyperbolic Coxeter polytopes with Gale diagram G 1411. Their Coxeter diagrams are shown in the upper row of Table 4.9. the electronic journal of combinatorics 14 (007), #R69 16

Proof. Let P be a compact hyperbolic Coxeter polytope with Gale diagram G 1411. This Gale diagram contains an arc J = 1, 4, 1. Hence, the Coxeter diagram Σ of P contains a diagram Σ J which coincides with one of the three diagrams shown in Item of Table 3.. Further, Σ contains two Lannér diagrams of order 3, one of which (say, L) intersects Σ J. Denote the common node of that Lannér diagram L and Σ J by, the 5 remaining nodes of Σ J by u,..., u 6 (in a way that u 6 is marked white in Table 3., i.e. it belongs to only one Lannér diagram of order 5), and denote the two remaining nodes of L by u 7 and u 8. Since L is connected, we may assume that u 7 is joined with. Notice that is also a node marked white in Table 3., elsewhere it belongs to at least three Lannér diagrams in Σ. Consider the diagram Σ = Σ J, u 7. It is connected, and all Lannér diagrams contained in Σ are contained in Σ J. In particular, Σ does not contain neither dotted nor multi-multiple edges. Hence, we have only finite number of possibilities for Σ. More precisely, to each of the three diagrams Σ J shown in Item of Table 3. we must attach a node u 7 without making new Lannér (or parabolic) diagrams, and all edges must have PSfrag replacements multiplicities at most 3. In addition, PSfrag u 7 is joined replacements with. The last condition is restrictive, since we know that and u 6 are the nodes of Σ J marked white in Table 3.. A direct computation (using the technique described in Section 3.) leads us to the two diagrams Σ 1 and Σ (up to permutation of indices, 3, 4 and 5 which does not play any role) shown in Fig. 4.4. u 5 u 3 u u 7 u 5 u 3 u u 7 Σ 1 = Σ = u 8 u 6 Figure 4.4: Two possibilities for diagram Σ, see Lemma 4.4 u 8 u 6 Now consider the diagram Σ = Σ, u 8 = Σ J, u 7, u 8 = Σ J, L. As above, u 8 may attach to Σ J by edges of multiplicity at most 3, so the only multi-multiple edge that may appear in Σ is u 8 u 7. Since both diagrams Σ 1 and Σ have signature (6, 1), the corresponding vectors in R 6,1 form a basis, so the multiplicity of the edge u 8 u 7 is completely determined by multiplicities of edges joining u 8 with the remaining nodes of Σ. Thus, there is a finite number of possibilities for Σ. To reduce the computations note that u 8 is not joined with u, u 3,, u 5 (since the diagram u, u 3,, u 5, u 8 must be elliptic). Attaching u 8 to Σ, we do not obtain any diagram with zero determinant and prescribed Lannér diagrams. Attaching u 8 to Σ 1, we obtain the two diagrams Σ 1 and Σ shown in Fig. 4.5. The remaining node of Σ, namely u 9, is joined with u 6 by a dotted edge. It is also contained in a Lannér diagram u 7, u 8, u 9 of order 3, but no other Lannér diagram contains u 9. Since u 7 attaches to, we see that all edges joining u 9 with Σ \ u 6 are neither dotted nor multi-multiple. On the other hand, for both diagrams Σ 1 and Σ, the diagram Σ \ u 6 has signature (6, 1). Hence, the weight of edge u 9 u 8 is completely determined by multiplicities of edges joining u 9 with the remaining nodes of Σ \ u 6, so we are left the electronic journal of combinatorics 14 (007), #R69 17

PSfrag replacements u 5 u 3 u Σ 1 = u 6 u 7 u 8 Σ = Figure 4.5: Two possibilities for diagram Σ, see Lemma 4.4 PSfrag replacements u 5 u 3 u u 6 u 7 u 8 with finitely many possibilities for Σ \ u 6. Again, we note that u 9 is not joined with u, u 3,, u 5. Now we attach u 9 to and to u 7 by edges of multiplicities from 0 (i.e. no edge) to 3, and then compute the weight of the edge u 9 u 8 to obtain det(σ \ u 6 ) = 0. This weight is equal to cos π for integer m only in case of the diagrams shown in Fig. 4.6. m u 5 u 3 u u 7 u 8 u 5 u 3 u u 7 u 6 u 9 u 6 u 8 u 9 Figure 4.6: Coxeter diagrams of Coxeter polytopes with Gale diagram G 1411 The last step is to find the weight of the dotted edge u 9 u 6 to satisfy the signature condition, i.e. the signature should equal (6, 1, ). We write the determinant of Σ as a quadratic polynomial of the weight d of the dotted edge, and compute the root. An easy computation shows that for both diagrams the signature of Σ for d = 1+ 5 is equal to (6, 1, ), so we obtain that these two diagrams correspond to Coxeter polytopes. One can note that the right polytope can be obtained by gluing two copies of the left one along the facet corresponding to the node u 8. Lemma 4.5. There are no compact hyperbolic Coxeter polytopes with Gale diagram G 5. Proof. Suppose that there exists a hyperbolic Coxeter polytope P with Gale diagram G 5. The Coxeter diagram Σ of P contains a Lannér diagram L 1 =,... u 5 of order 5, and two diagrams of order, denote them L = u 6, u 8 and L 3 = u 7, u 9. The diagram L 1, L is connected, otherwise it is superhyperbolic. Thus, we may assume that u 6 attaches to L 1. Similarly, we may assume that u 7 attaches to L 1. Therefore, the diagram Σ = L 1, u 6, u 7 consists of a Lannér diagram L 1 of order 5 and two additional nodes which attach to L 1, and these nodes are not contained in any Lannér diagram. According to [E1, Lemma 3.8] (see Tabelle 3, page 7, the case N F =, L F = 5), Σ must coincide with the diagram (up to permutation of indices of nodes of L 1 ) shown in Fig. 4.7. Consider the diagram Σ 1 = Σ, u 8 = Σ\u 9. The node u 8 is joined with u 6 by a dotted edge. The diagram Σ 1 \ u 6 contains a unique Lannér diagram, L 1. If u 8 attaches to L 1, Σ 1 \ u 6 should coincide with Σ. Thus, u 8 does not attach to,...,, and [u 8, u 5 ] = or 3. It is also easy to see that [u 8, u 7 ] 4. Since the signature of Σ is (6, 1), the weight the electronic journal of combinatorics 14 (007), #R69 18

PSfrag replacements u u 3 u 5 u 6 Figure 4.7: The diagram Σ, see Lemma 4.5 u 7 of the edge u 8 u 6 is completely determined by multiplicities of edges joining u 8 with the remaining nodes of Σ. Hence, we have a finite number of possibilities for Σ 1. To reduce the computations observe that either [u 5, u 8 ] or [u 7, u 8 ] must equal. We are left with only 4 cases: the pair ([u 5, u 8 ], [u 7, u 8 ]) coincides with one of (, ), (, 3), (, 4) or (3, ). For each of them we compute the weight of u 8 u 6 by solving the equation det Σ 1 = 0. Each of these equations has one positive and one negative solution, but the positive solution in case of ([u 5, u 8 ], [u 7, u 8 ]) = (, 4) is less than one, so it cannot be a weight of a dotted edge. Therefore, we have three cases ([u 5, u 8 ], [u 7, u 8 ]) = (, ), (, 3) or (3, ), for which 4+ 5 the weight of u 8 u 6 is equal to 11, 3 5+7+4 4 5 9+5, and 5+4 5 respectively. 5 11 By symmetry, we obtain the same cases for the diagram Σ = Σ, u 9 = Σ \ u 8, and the same values of the weight of the edge u 9 u 7 when ([u 5, u 9 ], [u 6, u 9 ]) = (, ), (, 3) and (3, ) respectively. Now, we have only 9 cases to attach nodes u 8 and u 9 to Σ (in fact, there are only six up to symmetry). For each of these cases we compute the weight of the edge u 8 u 9 by solving the equation det Σ = 0. None of these solutions is equal to cos π for integer m, which contradicts the fact that the diagram u m 8, u 9 is elliptic. This contradiction proves the lemma. 4.3 Dimension 5 In dimension 5 we must consider six Gale diagrams, namely G 4, G 33, G 1311, G 1311, G 11411, and G 11. Lemma 4.6. There is only one compact hyperbolic Coxeter polytope with Gale diagram G 11. Its Coxeter diagram is the left one shown in the first row of Table 4.. PSfrag replacements Proof. The proof is similar to the proof of Lemma 4.3. We assume that there exists a hyperbolic Coxeter polytope P with Gale diagram G 11. This Gale diagram contains an arc J =,,. According to Lemma 3.1 (Item of Table 3.) and Lemma., the Coxeter diagram Σ of P consists of the subdiagram Σ J shown in Fig. 4.8, and two u u 3 u 5 u 6 Figure 4.8: A unique diagram Σ J for J =,, nodes u 7, u 8 joined by a dotted edge. By Lemma.1, the subdiagrams u 7,, u and the electronic journal of combinatorics 14 (007), #R69 19

u 5, u 6, u 8 are Lannér diagrams, and no other Lannér subdiagram of Σ contains u 7 or u 8. So, we need to check possible multiplicities of edges incident to u 7 and u 8. Again, we consider the diagram Σ = Σ J, u 7. It is connected, does not contain dotted edges, and its determinant is equal to zero. Furthermore, observe that u 7 does not attach to u, u 3,, u 5 (since the diagram u 7, u, u 3,, u 5 should be elliptic), and u 7 does not attach to u 6 (since the diagram u 7,, u 5, u 6 should be elliptic). Therefore, u 7 is joined with only. Solving the equation det Σ = 0, we find that [u 7, ] = 4. By symmetry, we obtain that u 8 is not joined with, u, u 3,, u 5, and [u 8, u 6 ] = 4. Thus, we have PSfrag the replacements Coxeter diagram Σ shown in Fig. 4.9. Assigning the weight d = u 3 u u 5 u 6 u 7 u 8 Figure 4.9: Coxeter diagram of a unique Coxeter polytope with Gale diagram G 11 ( 5 + 1)/4 to the dotted edge, we see that the signature of Σ is equal to (5, 1, ), so we obtain that this diagram corresponds to a Coxeter polytope. Before considering the diagram G 11411, we make a small geometric excursus, the first one in this purely geometric paper. The combinatorial type of polytope defined by Gale diagram G 11411 is twice truncated 5-simplex, i.e. a 5-simplex in which two vertices are truncated by hyperplanes very close to the vertices. If we have such a polytope P with acute angles, it is easy to see that we are always able to truncate the polytope again by two hyperplanes in the following way: we obtain a combinatorially equivalent polytope P ; the two truncating hyperplanes do not intersect initial truncating hyperplanes and intersect exactly the same facets of P the initial ones do; the two truncating hyperplanes are orthogonal to all facets of P they do intersect. The difference between polytopes P and P consists of two small polytopes, each of them is combinatorially equivalent to a product of 4-simplex and segment, i.e. each of these polytopes is a simplicial prism. Of course, it is a Coxeter prism, and one of the bases is orthogonal to all facets of the prism it does intersect. All such prisms were classified by Kaplinskaja in [K]. Simplices truncated several times with orthogonality condition described above were classified by Schlettwein in [S]. Twice truncated simplices from the second list are the right ones in rows 1, 3, and 5 of Table 4.. Therefore, to classify all Coxeter polytopes with Gale diagram G 11411 we only need to do the following. We take a twice truncated simplex from the second list, it has two the electronic journal of combinatorics 14 (007), #R69 0

right facets, i.e. facets which make only right angles with other facets. Then we find all the prisms that have right base congruent to one of right facets of the truncated simplex, and glue these prisms to the truncated simplex by right facets in all possible ways. The result is presented in Table 4.. All polytopes except the left one from the first row have Gale diagram G 11411. The polytopes from the fifth row are obtained by gluing one prism to the right polytope from this row, the polytopes from the third and fourth rows are obtained by gluing prisms to the right polytope from the third row, and the polytopes from the first and second rows are obtained by gluing prisms to the right polytope from the first row. The number of glued prisms is equal to the number of edges inside the maximal cycle of Coxeter diagram. Hence, we come to the following lemma: Lemma 4.7. There are 15 compact hyperbolic Coxeter 5-polytopes with 8 facets with Gale diagram G 11411. Their Coxeter diagrams are shown in Table 4.. Proof. In fact, the lemma has been proved above. Here we show how to verify the previous considerations without any geometry and without referring to classifications from [K] and [S]. Since the procedure is very similar to the proof of Lemma 4.6, we provide only a plan of necessary computations without details. Let P be a compact hyperbolic Coxeter polytope P with Gale diagram G 11411. This Gale diagram contains an arc J = 1, 4, 1, so the Coxeter diagram Σ of P consists of one of the diagrams Σ J presented in Item of Table 3. and two nodes u 7 and u 8 joined by a dotted edge. Choose one of three diagrams Σ J. Consider the diagram Σ = Σ J, u 7. It is connected, contains a unique dotted edge, no multi-multiple edges, and its determinant is equal to zero. So, we are able to find the weight of the dotted edge joining u 7 with Σ J depending on multiplicities of the remaining edges incident to u 7. The weight of this edge should be greater than one. Of course, we must restrict ourselves to the cases when non-dotted edges incident to u 7 do not make any new Lannér diagram together with Σ J. The number of such cases is really small. Further, we do the same for the diagram Σ = Σ J, u 8, and we find all possible such diagrams together with the weight of the dotted edge joining u 8 with Σ J. Then we are left to determine the weight of the dotted edge u 7 u 8 for any pair of diagrams Σ and Σ. It occurs that this weight is always greater than one. Doing the procedure described above for all the three possible diagrams Σ J, we obtain the complete list of compact hyperbolic Coxeter 5-polytopes with 8 facets with Gale diagram G 11411. The computations completely confirm the result of considerations previous to the lemma. In the remaining part of this section we show that Gale diagrams G 4, G 33, G 1311, and G 1311 do not give rise to any Coxeter polytope. Lemma 4.8. There are no compact hyperbolic Coxeter polytopes with Gale diagram G 1311. the electronic journal of combinatorics 14 (007), #R69 1

Proof. Suppose that there exists a compact hyperbolic Coxeter polytope P with Gale diagram G 1311. This Gale diagram contains an arc J =, 3, 1. According to Lemma 3.1 (Item 9 of Table 3.) and Lemma., the Coxeter diagram Σ of P consists of one of the nine subdiagrams Σ J shown in Table 4.1, and two nodes u 7, u 8 joined by a dotted edge. Table 4.1: All possible diagrams Σ J for J =, 3, 1 ag replacements u u 5 u 3 u 6 3,4,5 u u 5 u 3 u 6 u u 5 u 3 u 6 u u 5 u 3 u 6 4,5 u u 5 u 3 u u 5 u 6 u 6 u 3 By Lemma.1, the subdiagrams u 7,, u and u 6, u 8 are Lannér diagrams, and no other Lannér subdiagram of Σ contains u 7 or u 8. Consider the diagram Σ = Σ J, u 7. It is connected, does not contain dotted edges, and its determinant is equal to zero. Observe that the diagram u, u 3,, u 5 is of the type H 4. Since the diagram u 7, u, u 3,, u 5 is elliptic, this implies that u 7 is not joined with u, u 3,, u 5. Furthermore, notice that the diagram u 3,, u 6 is of the type H 3. Since the diagram u 7, u 3,, u 6 is elliptic, we obtain that [u 7, u 6 ] = or 3. Thus, for each of 9 diagrams Σ J we have possibilities of attaching u 7 to Σ J \. Solving the equation det Σ = 0, we compute the weight of the edge u 7. In all 18 cases the result is not of the form cos π for positive integer m, which proves the lemma. m Lemma 4.9. There are no compact hyperbolic Coxeter polytopes with Gale diagram G 1311. Proof. Suppose that there exists a hyperbolic Coxeter polytope P with Gale diagram G 1311. This Gale diagram contains an arc J = 1, 3, 1. Therefore, the Coxeter diagram Σ of P contains one of the five subdiagrams Σ J, shown in Item 8 of Table 3.. On the other hand, Σ contains a Lannér diagram L of order 3 intersecting Σ J. Denote by the intersection node of L and Σ J, and denote by u 6 and u 7 the remaining nodes of L. Since L is connected, we may assume that u 6 attaches to. Denote by u the node of Σ J different from and contained in only one Lannér diagram of order 4, and denote by u 3,, u 5 the nodes of Σ J contained in two Lannér diagrams of order 4. Consider the diagram Σ 0 = Σ J, u 6 \ u. It is connected, has order 5, and contains a unique Lannér diagram which is of order 4. All such diagrams are listed in [E1, Lemma 3.8] (see the first two rows of Tabelle 3, the case N F = 1, L F = 4). We have reproduced this list in Table 3.3. the electronic journal of combinatorics 14 (007), #R69

Consider the diagram Σ 1 = Σ J, u 6 = Σ J, Σ 0. Comparing the lists of possibilities for Σ J and Σ 0, it is easy to see that Σ 1 coincides with one of the four diagrams listed PSfrag replacements in Table 4. (up to permutation of indices 3, 4 and 5). Now consider the diagram Σ = Table 4.: All possibilities for diagram Σ 1, see Lemma 4.9 u u 5 u 3 u 6 u u 5 u 6 4, 5 u 6 u 5 u 3 u u 3 Σ J, L = Σ 1, u 7. It is connected, does not contain dotted edges, its determinant is equal to zero, and the only multi-multiple edge may join u 7 and u 6. To reduce further computations notice, that the diagram u 7, u 3,, u 5 is elliptic, so u 7 does not attach to u 3,, and may attach to u 5 by simple edge only. Moreover, since the diagrams u 7, u,, u 5 and u 7,,, u 5 are elliptic, u 7 is not joined with u 5. Furthermore, since the diagrams u 7,,, u 5 and u 7,, u 3, are elliptic, [u 7, ] = or 3. Considering PSfrag elliptic replacements diagrams u 7, u,, u 5 and u 7, u, u 3,, we obtain that [u 7, u ] is also at most 3. Then for all 4 diagrams Σ 1 and all admissible multiplicities of edges u 7 and u 7 u we compute the weight of the edge u 7 u 6. We obtain exactly two diagrams Σ where this weight is equal to cos π for some positive integer m, these diagrams are shown in m Fig. 4.. We are left to attach the node u 8 to Σ. Consider the diagram Σ = Σ \ u. u u 5 u 6 u 7 u u 5 u 6 u 3 u 3 u 7 Figure 4.: All possibilities for diagram Σ, see Lemma 4.9 As usual, it is connected, does not contain dotted edges, its determinant is equal to zero, and the only multi-multiple edge that may appear is u 8 u 7. Furthermore, the diagram u 3,,, u 6 is of the type H 4, and the diagram u 8, u 3,,, u 6 is elliptic. Thus, u 8 does not attach to u 3,,, u 6. The diagram u 3,, u 5 is of the type H 3, and since the diagram u 8, u 3,, u 5 should be elliptic, this implies that [u 8, u 5 ] = or 3. Now for both diagrams Σ \ u Σ we compute the weight of the edge u 8 u 5. In all four cases this weight is not equal to cos π for any positive integer m, that finishes the proof. m Lemma 4.. There are no compact hyperbolic Coxeter polytope with Gale diagram G 33. Proof. Suppose that there exists a hyperbolic Coxeter polytope P with Gale diagram G 33. The Coxeter diagram Σ of P consists of two Lannér diagrams L 1 and L of order 3, and one Lannér diagram L 3 of order. Any two of these Lannér diagrams are joined in Σ, and any subdiagram of Σ not containing one of these three diagrams is elliptic. the electronic journal of combinatorics 14 (007), #R69 3